Electrical components, for example, resistors, transistors, and capacitors, are commonly mounted on circuit panel structures such as printed circuit boards. Circuit boards ordinarily include a generally flat sheet of dielectric material with electrical conductors disposed on a major flat surface of the sheet, or on both major surfaces. The conductors are commonly formed from metallic materials such as copper and serve to interconnect the electrical components mounted to the board.
Microelectronic circuit packages are prepared in various sizes. One packaging level includes semiconductor chips containing multiple microcircuits and/or other components. Such chips are usually made from semiconductors such as silicon, and the like. Intermediate package levels, also called chip scale packages (CSP) may include multi-layer substrates and a plurality of chips. These intermediate package levels can be attached to larger scale circuit cards, motherboards, and the like. The intermediate package levels serve several purposes in the overall circuit assembly including structural support, transitional integration of the smaller scale circuits to larger scale boards, and the dissipation of heat from the circuit components. Substrates used in conventional intermediate package levels have included a variety of materials, for example, ceramics, fiberglass reinforced polyepoxides, and polyimides.
There has been an increasing need for circuit panel structures, which provide high density, complex interconnections. Such a need can be addressed by multi-layer circuit panel structures; however, the fabrication of such multi-layer circuit assemblies has presented serious drawbacks. It is difficult to align holes and vias by drilling the dielectric layers as increasing layers of circuitry are applied. Laser drilling is the most common method for forming vias, which can add significant cost to manufacture such circuit assemblies.
Generally multi-layer panels are made by providing individual, dual sided circuit panels with appropriate conductors thereon. The panels are then laminated one atop the other with one or more layers of uncured or partially cured dielectric material, commonly referred to as “prepregs” disposed between each pair of adjacent panels. Such a stack ordinarily is cured under heat and pressure to form a unitary mass. After curing, holes typically are drilled through the stack at locations where electrical connections between different boards are desired. The resulting holes or “through vias” are then coated or filled with electrically conductive materials usually by plating the interiors of the holes to form a plated through via. It is difficult to drill holes with a high ratio of depth to diameter, thus the holes used in such assemblies must be relatively large and consume a great deal of space in the assembly.
In applications wherein circuit layers are built one on top of another, a dielectric material typically separates the circuitized layers. Polymeric dielectric materials that typically are used in circuit assembly, are thermoplastic or thermoset polymers. Thermoset materials are typically cured first to form a conformal coating. Although the conformally coated substrate may contain through holes conforming to a perforate substrate, blind vias are typically formed by drilling, such as by a laser.
It would be desirable to provide a method of forming solid blind vias or raised conductive structures in substrates using fabrication techniques that reduce the costs of via formation compared to mechanical and laser drilling techniques.